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From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/107541] [13 Regression] wrong code at -O1,
 -O2 and -O3 on x86_64-linux-gnu
Date: Mon, 07 Nov 2022 07:57:19 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: needs-bisection, wrong-code
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107541

--- Comment #4 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
This is an issue with the TRUNC_DIV_EXPR range-op entry optimizing division=
s by
powers of 2 into right shifts.  We're right shifting the mask by the shift
amount.

operator_div::fold_range():
...
...
  tree t;
  if (rh.singleton_p (&t))
    {
      wide_int wi =3D wi::to_wide (t);
      int shift =3D wi::exact_log2 (wi);
      if (shift !=3D -1)
        {
          wide_int nz =3D lh.get_nonzero_bits ();
          nz =3D wi::rshift (nz, shift, TYPE_SIGN (type));
          r.set_nonzero_bits (nz);
        }
    }

The operands are:
[irange] int [-256, -255] NONZERO 0xffffff01
[irange] int [8, 8] NONZERO 0x8

Result before optimization:
[irange] int [-32, -31] NONZERO 0xffffffe1

Result after the optimization:
[irange] int [-32, -31] NONZERO 0xffffffe0

I'll take a look.=